1. Field of the Invention
This invention relates to a tool and, more particularly, to an adjustable length drill bit extension.
2. Prior Art
Extendible tool attachments for use with hand drills and the like are well known in the art. Extendible tool holders are required, for example, when it is necessary to drill a hole through a stud or a pair of studs wherein the space available for inserting the drill and the bit may be restricted and the material thickness is greater than the length of the drill bit. In such instances, it is common to select one of a plurality of fixed-length drill bits from a set, each drill bit in the set having a different shank length. As a further example, circular hole cutting bits having a large (one to two inch) outer diameter have a shaft attached axially thereto, which shaft has a proximal end adapted to be received and securely held within a chuck or spindle of a drill, are commercially available in a variety of shank lengths.
Notwithstanding the current commercial availability of drill bits having various shank lengths, it is particularly desirable to have a single drill bit with a variable shank-length wherein the drill bit is operable for drilling a hole having a variable depth and a single preferred diameter through a varied thickness of material such as wood. The drill bit should preferable have a telescopically axially adjustable shank length so that it can drill through various thickness' of wood. Such drill bit extensions have been disclosed in the art as shown, for example, in U.S. Pat. Nos. 1,376,139; 918,062; and 1,225,209.
While such adjustable drill bit extensions are known, the use of such extendible shafts is limited because the adjustable shank-length construction does not permit high torque applications. That is, the extension potion of the shank of prior art devices employs a cylindrical shaft telescopically mounted within the cylindrical bore of a sleeve. Such a tool fails upon application of high torque. It is therefore desirable to provide an extendible shaft or shank for a drill bit wherein the shank construction permits the use of the bit in a high torque application.